The NCEP/NCAR Reanalysis 1 project is using a state-of-the-art analysis/forecast
system to perform data assimilation using past data from 1948 to the present.
A large subset of this data is available from PSD in its original 4 times
daily format and as daily averages. However, the data from 1948-1957 is
a little different, in the regular (non-Gaussian) gridded data. That data
was done at 8 times daily in the model, because the inputs available in that
era were available at 3Z, 9Z, 15Z, and 21Z, whereas the 4x daily data has
been available at 0Z, 6Z, 12Z, and 18Z. These latter times were forecasted
and the combined result for this early era is 8x daily. The local
ingestion process took only the 0Z, 6Z, 12Z, and 18Z forecasted values, and
thus only those were used to make the daily time series and monthly means here.

Please note: If you acquire NCEP Reanalysis Derived
data products from PSD, we ask that you acknowledge us in your use of the
data. This may be done by including text such as
NCEP Reanalysis Derived data provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their Web site at
http://www.esrl.noaa.gov/psd/ in any documents or publications using
these data. We would also appreciate receiving a copy of the relevant
publications. This will help PSD to justify keeping the
NCEP Reanalysis Derived data set freely available online in the
future. Thank you!